As the system of directly plate-making from digital signals such as computer data, systems by (1) electrophotography, (2) photopolymerization by combination of exposure by Ar laser and post-heating, (3) lamination of a silver salt photosensitive material on a photosensitive resin, (4) a silver master type, and (5) breakdown of a silicone rubber layer by discharge breakdown and laser beams are known.
However, a method of using electrophotography of (1) is complicated in processes such as electrification, exposure and development, and requires intricate and large-scaled equipment. A method (2) requires highly sensitive printing materials over and above a post-heating process, and handling in daylight is difficult. Since methods (3) and (4) use silver salts, processing is complicated and uneconomical. A method (5) is a method of comparatively reaching completion, but a problem of the elimination of silicone residues on a plate remains. On the other hand, the development of laser in recent years is remarkable, in particular, small-sized solid state lasers and semiconductor lasers having emission regions in near infrared to infrared of high output are easily available. Laser is very useful as the exposure light source of direct plate-making from digital signals such as computer data.
As conventional lithographic printing plate materials, Japanese Patent Publication No. 27919/1971 discloses a method of forming an image by heating according to data a printing material containing a recording layer to which a polymer compound or a composition which is insoluble or slightly soluble before heating and becomes more soluble in a solvent under heating is mixed, and Japanese Patent (Application) Laid-Open No. 69192/1981 discloses a heat-sensitive recording material having a heat-sensitive layer containing a novolak type phenol resin and a carbon black. However, these patents disclose only the examples of the case where images are recorded without using laser beams, and when images are recorded by lasers having emission regions from near infrared to infrared as the exposure light sources in the case of directly plate-making from digital data such as computers, good printed matters cannot necessarily be obtained due to background stain (i.e., scum) and the reduction of impression capability. For obtaining good printed matter, it is necessary that an exposed area (a non-image area) should be easily dissolved and an unexposed area (an image area) remain by alkali development processing after exposure and the remaining image area have good durability. That is, it is thought that since the recording property of the image is not good, the non-image area is dissolved with difficulty and the image area is easily dissolved when laser beams are used in the above well-known techniques.
To cope with these problems by conventional techniques, Japanese Patent (Application) Laid-Open No. 285275/1995 discloses to use, as the image-forming layer, an image-recording material containing a binder, a substance which absorbs light and generates heat, and a substance which is heat-decomposable and substantially reduces the solubility of the binder in the state not being decomposed, to thereby obtain a lithographic printing plate material having good printability capable of directly plate-making from digital signals such as computer data with conventional processing apparatus and printing machine as they are.
However, when the material is processed with a fatigued developing solution lowered in activity, the processability (development latitude) is not sufficient even with the technique disclosed in Japanese Patent Laid-Open No. 285275/1995.